plate-covers on the top, the bottom being latticed. The web consists of a double system of main struts and tension-bars inclined at 45°, and connected at their intersection midway between the chords by pins. From this point a vertical sub-post extends upward to support the middle of the panel of the upper chord. The splices are simple butt joints strengthened by cover-plates. The braces are "channels" latticed on two sides; the ends are reinforced, and they are bored for upper, middle, and lower pin-connections. Members subjected only to tension are made of eye-bars, as in bottom chords (figs. 20, 42). The lateral struts and rods are attached directly to the pins by U-shaped nuts, and a vertical bolt dispensing with the use of cast iron. The cross-ties of oak, 7 x 14, were laid directly upon the upper chords, which were proportioned to resist this extra bending strain. The ties were spaced 21 inches apart. During the progress of the work the average number of daily trains was sixty-four, yet no interruption to traffic occurred. This form of truss has been extensively introduced by its engineers. It is light, strong, and economical. (See fig. 42.) The longest single-span trussed girder in existence is believed to be that crossing the Ohio River at Cincinnati on the Cincinnati Southern Railroad; whilst one of the boldest and most economically constructed trusses in American experience is the continuous girder of three spans crossing the Kentucky River on the same railroad. These bridges are described by Mr. T. C. Clarke, C. E.: "The bridges of the Ohio have to be constructed in accordance with acts of Congress of the United States, fixing their minimum spans and heights, and all the plans have to be approved by the U. S. engineers. The requisitions laid down by Congress demand a channel-span of 500 feet in the clear between the bases of the piers, or 515 feet between the points of support, placed at a height of 105 feet above low water in the river, which is 10 feet deep, while the river rises 62 feet in times of floods. "The lower line of staging of the bridges at Cincinnati was erected in December, 1876. The surface of the river was covered with ice. A sudden thaw and rise of water of 15 feet washed away the lower staging. Operations were suspended until June, 1877, when the staging was rebuilt. The bottom of the timber trestles stood on the rocky bed of the river. About 500 feet above the line of the bridge a timber crib was sunk and filled with stone. From this a fender of coal-barges chained together was let down to each pier, protecting the trestle from being struck by boats or drift-wood. "The iron-work of this channel-span of 515 feet had been submerged by a freshet and required washing. The iron was cleaned, elevated 100 feet by steam-power, run forward over two spans of 300 feet each, and then hoisted into position by two travellers on the top staging worked by hand. This was all done in 24 working days by an average force of 60 men. The span was swung clear of its staging by Full Panel, upper chord. knocking out folding wedges, and then required 30 men for six days more to adjust it, put on cross-girders and track-stringers, and lay the ties and guard-timbers. "The span was tested by running over it seven locomotives and four loaded platform cars, their combined weight amounting to 431 tons. The centre deflection of the east truss was 2 inches; the permanent set was inch. The centre deflection of the west truss was 2 inches; the permanent set was none. "The cost of this span was $209,500. This bridge was designed and executed by Mr. J. H. Linville, C. E. "The Kentucky River bridge consists of three spans of 375 feet each, carrying the Cincinnati Southern Railway across a limestone cañon at a height of 280 feet above the bed of the stream. The piers are of stone to a height of 60 feet, which carries them a little above the highest recorded floods, and of iron for the rest of their height. As the floods are very sudden, it was decided to adopt a plan of construction obviating 'false works' or stagings of timber, which might be swept away by freshets or knocked down by rafts, which pass by during floods at a speed of 7 or miles per hour. "In 1854, Mr. J. A. Roebling had begun to build a suspension bridge of 1236 feet span across this chasm, but the work was abandoned for want of funds after two towers and two Date of erection. space, and by midday the lower chords had expanded until the gap in the east chord was closed, which was then riveted up, and by a similar process the gap in the west chord was closed. sets of anchorage had been constructed. Mr. C. Shaler Smith, the engineer whose design was adopted, took advantage of these towers by bolting the first panel of his bridge on each side to them, and then corbelling out panel by panel. The towers were calculated to be strong enough to carry 196 feet of projecting spans. At this point temporary towers of wood were built, which gave an intermediate support. The corbelling-out process was continued until the shore spans each reached the main iron piers, which were built up simultaneously, so that the two met in mid-air. These piers had been on rollers, as it was impossible to foresee the temperature at which the junction would take place. Each pier, weighing 200 tons, was moved horizontally until a junction was made. Each half of the centre span was then corbelled out as before until they met in the centre. At this stage of the work, the upper chords being in tension and the lower in compression, the former were nearer to each other than the latter, the gaps being upper chord, east gap, 3 inches; west gap, 2 inches; lower chord, east gap, 4 inches; west gap, 5 inches. The gap of 2 inches in the west upper chord was first closed by screwjacks, which had been placed between the ends of the lower chords and the abutments on the shore spans, and by moving the piers together. This left a gap of 1 inches between the ends of the east top chord. At mid-day, the temperature of the air being 70° Fahr., all the horizontal lateral rods tending to draw these ends together were screwed up and the counter slackened. The contraction of the lateral rods drew the gap together. On the follow-rated again after the hinge was passed. When the bridge ing morning, at a temperature of 40° Fahr., the gap had closed and the top chord connections were riveted up. The contraction due to temperature had by that time withdrawn the shore ends of the lower chords from the screw-jacks inch. These were then screwed home so as to take up this Chronological Table of Tubular and Girder Bridges for Single-track Railway, constructed of Iron Spans exceeding 300 feet. COMPILED BY T. C. CLARKE, C. E. "Up to this time this bridge was a girder of 1125 feet long, continuous over three spans; but it had been foreseen that a continuous girder would not answer for this situation, because, while the abutments on the cliffs were stationary, the iron piers would constantly rise and fall with changes of temperature, and so vary the strains on the web system. It was therefore determined to hinge the shore spans at points 75 feet from the piers, leaving a centre girder 525 feet long supported by piers 375 feet apart. By this means the shore spans were practically reduced to 300 feet each, one end resting on the abutment, and the other on the overhanging end or cantilever of the centre span. The final operation, therefore, was to cut the lower chords of the shore spans at points previously determined by calculation, at which points tenon-joints had been made and temporary rivets inserted. These rivets were cut out, and the mean motion of the several joints was only inch, and the change in the levels barely perceptible. This proves the accuracy of the method used to determine the point of contrary flexure, which was to work out the strains panel by panel as in calculating discontinuous spans. To avoid ambiguity in the web strains at the hinging points, both of the web systems of diagonal rods were consolidated into one member at the point of contrary flexure, and sepacame to be tested, it was found that the movement of the lower chord tenons under the passing load was 11 inches. This shows how great may be the strains concealed in the web system of a continuous girder of large span at the point of contrary flexure. tributing the iron. The total cost of erection, including wrought iron, and all others of wood, with cast- or plant, was $404,230. "Seven floods took place during the construction of the bridge, their rise varying from 56 to 47 feet. "This is not only one of the boldest and most original pieces of bridge-engineering in America, but, when judged by the crucial test of accomplishing a great deal at the least possible cost, it stands very high among engineering structures all over the world, and its design and execution reflect the highest credit upon its engineer, Mr. C. Shaler Smith, C. E." The highest truss bridge in the world is probably that on the Mont Cenis Railway, which spans a ravine in the Piedmontese Alps. It is known as the Comba Scura bridge, and has a height of 395 feet and a span of 185 feet 2 inches between the abutments. The bridge at El Kantara in Algeria has almost the same dimensions-viz., 393'6 feet high and 188 27 feet wrought-iron connections. In a valuable paper on this subject, Mr. C. L. Strobel, C. E., of Pittsburg, recommends the use of wrought iron for the struts near the centre of the bridge, as well as for the floor-beams, thus enabling the structure to be converted into one solely of iron when the wood shall have decayed. The design shown in fig. 43 is of the Warren type, and is adapted for spans of from 168 to 190 feet. It is said to be much more economical than any of the combination bridges in use at present, as those of Howe, Pratt, Post, and others. The floor-beams, of wood, are suspended from shoe connects the wooden post and strut with the eyethe pin at the panel and sub-panel points. A cast-iron bars by means of a pin. A similar form of connection is used in the wroughtiron "combination." The Post Bridge (fig. 44) is a combination of wood and iron, the former being used for the compression, the latter for the tension, members. The road-bearers, of wood, are supported upon plates suspended from pins at the panel-points of the lower chord. The posts are inclined at an angle of 39° to the vertical, and the ties have therefore a "run" of a panel and a half. The connections of posts and chords are made by means of cast-iron shoes placed upon the ends of the posts. These bridges are adapted to spans of 200 feet or less, and are still extensively used on railroads in the United States. STEEL BRIDGES.-But few steel bridges have been erected, owing to the limited knowledge of the best physical conditions of the material. Those in existence are of too recent date to furnish reliable data as to the ability of steel to fulfil all the requirements of strength, lightness, economy, durability, uniformity of texture, etc. The first and longest steel tubular arch in existence is the 522 39 feet span of the St. Louis bridge, erected between 1868 and 1874, and described under the section on Erection. This was soon succeeded by a steel quadrangular girder at Glasgow, Mo., and this in turn by a bridge opened Oct. 21, 1882, between Bismarck and Mandan, Dak. The bridge carries the Northern Pacific Railroad across the Missouri at an elevation of 100 feet above low water. It consists of three spans of 400 feet each, with a shore-span at either end of 125 feet (fig. 45). The western approach consists of a trestle 1600 feet long and about 65 feet high, built of wood and protected from ice and drift by dykes on both sides. The tests were successfully made on the date above mentioned, and their result reflected great credit on the engineer, Mr. George S. Morison. Cost, about $1,000,000. Among the latest British projects of engineering science may be noted the steel bridge over the Firth of Forth at Queensferry (fig. 46), having a total length of 5050 feet, and thus described in The American of May 10, 1882: "The bridge over the Firth of Forth will be one of the greatest marvels of engineering the world can show, as its two principal spans will be each 1700 feet, its two side-spans 675 feet, and the height above high water of the central 500 feet of the two main spans 200 feet. Any girder may conveniently be considered as a combination of two cantilevers with a central girder, and in this case the main spans will actually be built of two huge lattice cantilevers, each 675 feet long, and a central girder 350 feet long. The lower member of the cantilever, where it springs from the piers at 20 feet above high water, is a steel tube 12 feet in diameter and 2 inches thick, diminishing in the centre girder to a trough 3 feet deep. The upper chord of the cantilever slopes downward, while its lower chord is arched upward to permit of free navigation, so that the bridge has a varying height. The width will also vary at different points. diminishing from 120 feet over the piers (which are 150 feet long in the direction of the length of the bridge) to 27 feet in the centre girder. "That British railway engineers have largely ignored the force of the wind in their calculations was shown but too plainly by the fall of the Tay bridge during a fearful storm; but there appears to be no chance for such an FIG. 45.--Steel Bridge at Mandan, over the Missouri River, Northern Pacific R. R. of the steel employed in the structure. The depth of the | say, each bay of ties, struts, braces, etc., will be added to the water, 200 feet, compels 'erection by overhang;' that is to last, finished and braced against storms before the next is FIG. 46.-Proposed Bridge over the Firth of Forth, Scotland. commenced, till the immense cantilevers, like huge wings projecting from the piers, stand ready to receive their connecting girders." Trestle and Pile Bridges.-This form of bridge dates from the earliest period of which we have any authentic record, but modern appliances have rendered it so available in passing obstacles otherwise insurmountable that a brief review of the progress in this department cannot be omitted. Of wooden pile bridges the present (1883) practice is well exemplified by the annexed drawing (fig. 47) of the standard used on the Northern Pacific Railroad for structures over 14 feet in height. The first examples of cast-iron trestles of any magnitude were erected in 1853 by Mr. Fink on the Baltimore and Ohio Railroad. Two of these viaducts cross the Cheat River at an elevation of 250 feet on a grade of 2 per cent., and one of them on a curve of 800 feet radius. The trestle is 60 feet high, composed of posts in two sections, the diameters being 7 and 6 inches, with-inch thickness of metal. These viaducts are 500 feet long, built in lengths of 125 feet. They have been in use thirty years, and are still doing good service. About 1864, Mr. C. Shaler Smith substituted wrought iron for other materials in the construction of viaducts, using the Phoenix columns for posts. Each "bent consisted of two posts, with cross struts at intervals of 30 feet, united by diagonal tie-rods. The bents were spaced 30 feet apart and supported trussed girders carrying the roadway. This general system has now become very common, and is well adapted to suit the peculiar requirements of each case. It is well exemplified in the Lyman Viaduct, built by the Phoenix Bridge Co. on the Connecticut Air ШИ End Elevation. Side Elevation. FIG. 47.-Pile Bridges, Northern Pacific R. R. Line Railroad at East Hampton, Conn. This structure is 1100 feet long and 135 feet high. One of the most remarkable wooden trestles ever constructed, both on account of its height and the rapidity with which it was assembled, was that erected over Potomac Creek, Va., on the railroad from Acquia Creek to Fredericksburg during the late Civil War (fig. 48). Of this structure Major-Gen. McDowell, commanding Army of Potomac, testified as follows before a court of inquiry: "The Potomac Run Bridge, 400 feet long by 80 high, is a most remarkable structure. When it is considered that in the campaigns of Napoleon trestle bridges of more than one story were regarded as impracticable, and that, too, for common military purposes, it is not difficult to understand why distinguished Europeans should express surprise at so bold a specimen of American engineering. It is a structure which ignores all the rules and precedents of military science as laid down in books. It is constructed chiefly of round sticks cut from the woods, and not even divested of bark; the legs of the trestles are braced with round poles. It is in four stories-three of trestles and one of crib-work. It carries daily from ten to twenty heavy railway-trains in both directions, and has withstood several severe freshets and storms without injury. This bridge was built in May, 1862, in nine working days, during which time the greater part of the material was cut and hauled. It contains more than 2,000,000 feet of lumber. The original structure which this replaced required as many months as this did days. It was constructed by the common soldiers of the Army of the Rappahannock under the supervision of Gen. Herman Haupt, chief of the bureau of construction and transportation U. S. military railroads." In general dimensions, however, this timber struc FIG. 48.-U. S. Military Railway Department Trestle Bridge, across Potomac Creek on Fredericksburg R. R., Va. ture was surpassed by one erected under very different placed between each of the long spans. The trusses circumstances by skilled artisans and in a much longer are supported by wrought-iron columns, the ends of time. It is known as the Portage Bridge on the New two adjacent trusses resting upon a single column. York, Lake Erie, and Western Railroad, over the Gen- The pair of columns supporting the opposite trusses esee River, and is 800 feet long and 234 high, the ma- are in the same vertical plane, but are inclined towards sonry being 30 feet high, trestles 190, and trusses 14. each other and connected with wrought-iron struts 25 The timber piers, 50 feet apart, were estimated to sup- feet apart, and diagonal tie-rods, thus forming a twoport 3000 tons, besides the weight of the structure. post bent; each column is connected with the parallel The general plan was made by Silas Seymour, chief column of the adjoining bent by a similar arrangement engineer. This bridge was begun July 1, 1851, and of struts and diagonal ties; four columns with the concompleted Aug. 14, 1852. It was burned down May necting bracing are thus made to form a skeleton tower 6, 1875, but was immediately replaced by an iron 20 feet wide and 50 feet long on the top, surmounted trestle, which is quite as remarkable for the rapidity by a 50-feet span of bridge, having the same length at with which it was erected. (See fig. 49.) The descrip- the bottom and a width varying with the height of the tion is best given by the following extract from the tower. There are six of these towers, the largest havTransactions of the American Society of Civil Engi-ing a total height from masonry to rail of 203 feet 8 neers, 1876: "On Monday, May 10, 1875, the contract for the iron-work was let to the Watson Manufacturing Company of Paterson, N. J., the bridge to be built according to plans of George S. Morison, C. E. The first iron column was raised June 13th. On July 29th the iron was all in position; on the following day the track was laid across, and on Saturday, July 31st, forty-two working days, the bridge was tested and thrown open for traffic. The total weight of iron in the bridge is 1,310,000 pounds. The main principle of the plan may be said to be that which characterizes all American bridge-building, and is the leading difference between the works of American and European engineers-the concentration of the material into the least possible number of parts. "The iron viaduct has ten spans of 50 feet, two of 100 feet, and one of 118 feet, a 50-feet span being inches. "The trusses of the superstructure are proportioned to carry a moving load of 3000 pounds per running foot. and an excessive load of 5000 pounds per foot, with a maximum tensile strain of 10,000 pounds per square inch. The towers are built to carry a moving load of 5400 pounds per running foot (being designed for two tracks); they are also calculated to resist a windpressure at right angle to the bridge of 30 pounds per square foot, exerted on the entire surface of the struc ture, and of a train of cars, and one of 50 pounds per square foot exerted on the surface of the structure alone. "The columns rest upon cast-iron pedestals, those on the north side of the bridge being secured by dowels to a cast-iron plate sunk in the masonry, and those on the south side being placed on rollers rolling at right angles to the axis of the bridge. The columns are made in 25 |